1. Field of the Invention
The present invention relates to an optical disc type determining method and an optical disc device, capable of reliably determining a CD signal surface of a double-sided dual disc which has, on one side, a DVD signal surface for recording data compliant with a DVD standard and, on the other side, the CD signal surface for recording data compliant with a signal standard equivalent to a CD standard, and also capable of reliably determining a plurality of types of optical discs.
2. Description of the Related Art
Generally, in circular-plate-shaped optical discs, information signals such as music data, image data or computer data are highly densely recorded in a track spirally (or concentrically) formed on a signal surface of a substrate of the optical disc, and a desired track can be accessed at high velocity when a recorded track is played back, such that the optical discs are used in various ways.
The optical discs of this kind can be broadly classified into a playback-only type in which a reflection film of, for example, aluminum is attached onto a track composed of uneven pit lines to form a signal surface, and a record/playback type in which a recording film and a reflection film are sequentially attached onto a track composed of concave grooves and convex lands to form a signal surface.
Furthermore, the optical disc is rotatably mounted on a turntable provided in an optical disc device, and a laser beam emitted from an objective lens in an optical pickup enters the optical disc through a beam entrance surface of an optical disc substrate, and then this laser beam is applied in a spot form onto the signal surface located a predetermined distance away from the beam entrance surface, thereby carrying out recording or playback. There are a plurality of types of optical discs depending on the recording format of information signals to be recorded in the signal surface:
FIGS. 1A to 1D schematically show diagrams to explain the plurality of types of optical discs, wherein FIG. 1A shows a CD, FIG. 1B shows a DVD-SL having a single-layer type signal surface, FIG. 1C shows a DVD-DL having a double-layer type signal surface, and FIG. 1D shows a double-sided dual disc.
First, as shown in FIG. 1A, a compact disc (CD) 10 is formed into the shape of a circular plate having an optical disc substrate 11 of about 120 mm in diameter and a central hole of 15 mm in diameter, the thickness of the substrate being about 1.2 mm. A CD signal surface 12 recording, for example, music data in compliance with a CD format is formed with a wide track pitch about 1.2 mm away from a beam entrance surface 11a of this optical disc substrate 11. Further, a protective film 13 is attached onto the CD signal surface 12. Then, a first laser beam L1 at a wavelength of the order of 780 nm narrowed by an objective lens OB1 having a numerical aperture (NA) of about 0.45 enters the optical disc from the side of the beam entrance surface 11a of the optical disc substrate 11 and is applied to the CD signal surface 12.
Next, as shown in FIG. 1B, a DVD-SL (digital versatile disc-single layer) 20 having a single-layer type signal surface is formed into the shape of a circular plate having a total thickness of about 1.2 mm in which an optical disc substrate 21 having a thickness of about 0.6 mm and a reinforcing substrate 24 hating a thickness of about 0.6 mm are affixed together via an adhesive layer 23. A DVD signal surface 22 recording, for example, image data in compliance with a DVD format is formed about 0.6 mm away from a beam entrance surface 21a of the lower optical disc substrate 21 with a smaller track pitch and a higher density than those of the CD signal surface 12 of the CD 10. Then, a second laser beam L2 at a wavelength of the order of 650 nm narrowed by an objective lens OB2 having a numerical aperture (NA) of about 0.6 enters the optical disc from the side of the beam entrance surface 21a of the optical disc substrate 21 and is applied to the DVD signal surface 22.
Next, as shown in FIG. 1C, a DVD-DL (digital versatile disc-dual layer) 30 having a double-layer type signal surface is formed into the shape of a circular plate having a total thickness of about 1.2 mm in which first and second optical disc substrates 31, 35 each having a thickness of about 0.6 mm are affixed together via an adhesive layer 33. A first DVD signal surface 32 recording, for example, image data in compliance with the DVD format is formed about 0.6 mm away from a beam entrance surface 31a of the lower first optical disc substrate 31 in such a manner as to attach a semi-transmissive reflection film, and a second DVD signal surface 34 recording, for example, image data in compliance with the DVD format is formed on the upper second optical disc substrate 35 in proximity to the first DVD signal surface 32. Then, the second laser beam L2 at a wavelength of the order of 650 nm narrowed by the objective lens OB2 having a numerical aperture (NA) of about 0.6 enters the optical disc from the side of the beam entrance surface 31a of the first optical disc substrate 31 and is applied to the first DVD signal surface 32 or the second DVD signal surface 34.
Next, as shown in FIG. 1D, a double-sided dual disc 40 for recording or playing on both sides, which has only recently been developed, is formed into the shape of a circular plate having a total thickness of about 1.5 mm, wherein a first optical disc substrate 41 having a thickness of about 0.9 mm and a second optical disc substrate 45 having a thickness of about 0.6 mm are affixed together so that signal surfaces 42, 44 may be back to back to each other via an adhesive layer 43. A CD signal surface 42 which records, for example, music data in compliance with the CD format and which is equivalent to the CD signal surface 12 of the CD 10 is formed with a wide track pitch about 0.9 mm away from one beam entrance surface 41a on the side of the lower first optical disc substrate 41. A DVD signal surface 44 recording, for example, image data in compliance with the DVD format is formed about 0.6 mm away from the other beam entrance surface 45a on the side of the upper second optical disc substrate 45 with a smaller track pitch than that of the CD signal surface 42. Then, the first laser beam L1 at a wavelength of the order of 780 nm narrowed by the objective lens OB1 having a numerical aperture (NA) of about 0.45 enters the optical disc from the side of the beam entrance surface 41a of the first optical disc substrate 41 and is applied to the CD signal surface 42. On the other hand, after the dual disc 40 is inverted to the side of the second optical disc substrate 45, the second laser beam L2 at a wavelength of the order of 650 nm narrowed by the objective lens OB2 having a numerical aperture (NA) of about 0.6 is applied to the DVD signal surface 44 from the side of the beam entrance surface 45a of the second optical disc substrate 45.
Here, when a plurality of types of optical discs are selectively recorded or played in the same optical disc device, it is necessary to previously determine the plurality of types of optical discs in the optical disc device, and there is a method of determining the above-mentioned optical discs: the CD 10, the DVD-SL 20 having the single-layer type signal surface, and the DVD-DL 30 having the double-layer type signal surface (e.g., Japanese Patent Publication Laid-open No. 11-066712).
Now, according to the disc determining method disclosed in Japanese Patent Publication Laid-open No. 11-066712 mentioned above, although not shown here, the optical pickup is put into a CD playback state to carry out a focus search operation, and the difference between a maximum value and a minimum value of an obtained pickup output is defined as a first waveform level. The optical pickup is then put into a DVD playback state to carry out a focus search operation, and the difference between a maximum value and a minimum value of an obtained pickup output is defined as a second waveform level. The ratio between the first and second waveform levels is compared with a predetermined value to determine the types of the optical discs. Thus, when the plurality of types of optical discs different from each other in recording format are determined by the output of the optical pickup, the types of the optical discs are accurately determined even in the case of optical recording media such as a DVD and a CD that are about the same in reflectivity. It is also possible to reliably determine the single-layer/double-layer discs such as the above-mentioned DVD-SL 20 and DVD-DL 30.
However, in the above-mentioned double-sided dual disc 40 for recording or playing on both sides, the signal standard of the CD signal surface 42 is equivalent to the CD standard as described above, but the distance from the beam entrance surface 41a of the first optical disc substrate 41 to the CD signal surface 42 is about 0.9 mm (e.g., 0.87 mm, 0.89 mm, 0.93 mm) which is about 0.3 mm shorter than that in the CD standard. That is, the CD signal surface 42 of the dual disc 40 is located substantially in the midpoint between the CD signal surface 12 of the CD 10 about 1.2 mm away from the beam entrance surface 11a of the optical disc substrate 11 and the DVD signal surface 22 of the DVD-SL 20 about 0.6 mm away from the beam entrance surface 21a of the optical disc substrate 21 or the first, second DVD signal surface 32, 34 of the DVD-DL 30 about 0.6 mm away from the beam entrance surface 31a of the first optical disc substrate 31. Therefore, even if the technical idea of the disc determining method disclosed in Japanese Patent Publication Laid-open No. 11-066712 is applied, there arises a problem that the CD signal surface 42 of the dual disc 40 can not be identified or erroneously recognized.
Moreover, as the position from the beam entrance surface is difference between the CD signal surface 42 of the dual disc 40 and the CD signal surface 12 of the CD 10, there is a problem that normal playback can not be carried out in a CD standard setting and that much time is required for the adjustment of a focal position.